The Structure of Skin: Layers, Cells & Histology

 The skin is the largest organ of the body, functioning as a physical barrier while supporting immune defence, thermoregulation, and sensory input. Structurally, it consists of three main layers, each with unique cell types and functions.

📌 The Three Layers of Skin

🔹 Epidermis (Outer Layer)

- Composition: Stratified squamous epithelium in layers 

Layers of the Epidermis (Deep to Superficial):

• Stratum Basale (Germinativum): The deepest, single-cell layer attached to the dermis, where new keratinocytes are produced via mitosis. It also contains melanocytes for pigment and stem cells.
• Stratum Spinosum (Prickle Cell Layer): Comprised of 8 to 10 layers of keratinocytes that provide strength and flexibility, connected by desmosomes, providing mechanical strength to resist shearing forces.
• Stratum Granulosum (Granular Layer): Consists of 3 to 5 layers of cells that start to flatten and produce keratin granules (keratohyalin) and glycolipids, which help form a water-resistant barrier.
• Stratum Lucidum (Clear Layer): A thin, transparent layer present only in the thick skin of the palms and soles. Thick skin (palms and soles) has a prominent stratum lucidum and a thicker stratum corneum, whereas thin skin covers most of the body.
• Stratum Corneum (Horny Layer): 
• The outermost, waterproof layer composed of 15 to 30 layers of dead, flattened keratinocytes that are constantly shed, protecting against pathogens and dehydration. The layered organisation of the epidermis allows continuous renewal while maintaining a strong, water-resistant barrier.

• Damage to these layers compromises the skin’s barrier function and increases susceptibility to fluid loss and heat stress.

- Cell Types:

• Keratinocytes – The major structural cells, forming the protective keratinised barrier
• Melanocytes – Produce melanin for pigmentation and UV protection
• Langerhans cells – Antigen-presenting immune cells involved in skin surveillance

- Key Feature: The stratum corneum consists of dead, keratin-filled cells, constantly shedding and renewing (approx. every 28 days).

The intact epidermis limits transepidermal water loss, helping maintain fluid balance.

🔹 Dermis (Middle Layer)

- Composition: Dense irregular connective tissue

- Structures Inside:

• ✅ Blood vessels – Nourish the epidermis (which lacks its own blood supply)
• ✅ Nerve endings – Responsible for pain, pressure, temperature, and touch
• ✅ Sweat glands – Thermoregulatory eccrine glands, and apocrine glands linked to hair follicles
• ✅ Collagen & elastin fibres – Provide strength and elasticity

The extensive dermal capillary network allows heat exchange via vasodilation and vasoconstriction during thermoregulation. Eccrine sweat glands play a key role in thermoregulation through evaporative cooling.

🔹 Hypodermis / Subcutaneous Layer (Deepest Layer)

- Composition: Loose connective tissue & adipose

- Functions:

• ✅ Fat storage for energy reserve & insulation
• ✅ Shock absorption, protecting muscles & bones
• ✅ Houses major blood vessels & lymphatics

While the hypodermis provides insulation, excessive insulation can impair heat loss in hot environments.

Coming up: You’ll see how disruption of these layers affects thermoregulation and fluid balance when we look at skin function and sunburn in the next posts.

💡 Key Takeaway: The skin is a highly specialised, multilayered organ, each layer contributing to protection, sensation, and homeostasis.